CS4273: Distributed System Technologies and Programming I

CS4273: Distributed System Technologies and Programming I


Lecture 2: Introduction to Java and GUI

2

Important References for Java Manuals

•
Web site for Java Swing (GUI)


http://download.oracle.com/javase/tutorial/uiswing/


•
Web site for Java Swing Event handler


http://download.oracle.com/javase/tutorial/uiswing/events/index.html



•
Web site for Java 2 Standard Edition


http://www.oracle.com/technetwork/java/javase/overview/index.html


•
Sample Programs


Source codes of demo programs are in unix system: ~jia/www/java/

3

Java: an Object
-
Oriented Programming Language

Object
-
Oriented Programming:

•
Encapsulation

•
Inheritance

•
Polymorphism

4

Class and Method Definition

•
class

consists of

fields and methods:

class

class
-
name [
extends

superclass]



[
implements

interface] {




variable declarations;




method declarations;


}

•
method

declaration:


[modifiers] returnT method (args) {




statements;



}

public class dummyButton
extends

JApplet {


String font;


int style, size;


private JButton bold, italic;


public void init() {


bold = new JButton("BOLD");


add(bold);


add(italic = new JButton("ITALIC"));


font = "Helvetica";


style = Font.PLAIN;


size = 48;


}


public void paint(Graphics g) {


super.paint(g);


g.setFont(new Font(font, style, size));


g.drawString("Hello Java!", 50, 100);


}


class butnhandler
implements

ActionListener {


public void actionPerformed (ActionEvent e) {


………}

5

Object Initialization and Constructor

•
All variables must be initialized before use. Java automatically sets some
initial values for variables of the class, but not variables in methods.

•
A constructor is a special method, with the same name as the class, for
initialization. Java provides an empty & no
-
argument constructor if a class
does not have one.

class Rabbit {



int Age;




Rabbit

(int Age) { //constructor




this
.Age = Age; }//differentiate from the
Age

in the parameter




void run(int duration,boolean zigzag){ ... }

}

class RabbitGame {




public static void main(string[] args) {




Rabbit bunny = new
Rabbit
(3);




….

}

6

Class Inheritance and Method Overriding

•
When a method has the same
name as in the superclass, the
method overrides the one in
superclass. Otherwise the
superclass methods are
inherited with no change.

•
The top class in Java is
“object”. Every class is a
descendent of
object

(implicitly inherited).

•
Keyword
super

refers to the
superclass’ methods or
variables.

class Animals {


int Age;


Animals (int Age) { this.Age = Age; }




void wish ()


{ System.out.println ("I want to eat"); }

}

class Rabbit extends Animals {


.. .. ..

Rabbit (int Age, char Color) {


super

(Age);

// call super constructor


this.Color = Color;

}

void
wish
() {

// method over
-
ride


super
.wish();

// call a super method


System.out.println("I want a carrot"); }

}


Rabbit my_rabbit = new Rabbit(3, ‘B’);


my_rabbit.wish();

// ? output ?

7

Modifier: static

•
A variable or method defined
as
static

means that the
variable or the method belongs
to the class, and they are not
with objects.

•
Static variables / methods are
class variables / methods. There
is only one copy of them in the
system.

•
Class methods can only access
class variables. It cannot use
“this” reference, cannot call
normal methods, etc. But
normal methods can access
class variables.



class Rabbit {


static

int rabbits_count = 0;


int Age;


Rabbit (int Age) {


rabbits_count++;

// access static var



this
.Age = Age;


}


static

int count() {
// only use class variable


return rabbits_count;


}


}

object

object

Object Template

static vars
,

…

8

Modifiers: public, private, protected, final

•
public
: can be used everywhere.

•
private
: can be used only in its own class, not even from its subclass.

•
protected
: can be accessed within its own package.

•
final
defines:

–
a class can’t be extended

–
a method can’t be overridden

–
a variable cannot be modified (constant)


final

class Rabbit extends Animal {




final

float


= 3.14;





….





final

int run(int duration, boolean zigzag) { …. } // method 1






}

9

Abstract Methods and Abstract Classes

•
An
abstract

method is a method that has no implementation. It allows
subclasses to implement the method according to their own needs.

•
An
abstract

class is a class that contains at least one abstract method.

•
An abstract class must be FULLY implemented before it can be
instantiated.


abstract

class Animals {





int Age;




Animals (int Age) { this.Age = Age; }







abstract

void wish ();


}


class Rabbit extends Animals {



.. .. ..


void wish () {

// implement the abstract method



System.out.println("I want a carrot"); }


}

10

Interface and its Implementation

•
An interface is a class that all its methods are abstract.

•
There are many interfaces in Java, especially for event handlers, that
require implementations.

•
When defining a class which implements an interface, uses the format:


class xxx
implements

interface {




method declaration;




…….




method declaration;


}

•
If a class implements some methods of an interface, not all of them,
then this class becomes an abstract class.

11

Graphical User Interface (GUI)

Containers and Components

•
Containers provide a rectangular display area in which
components are positioned. A container can also contain
other containers.

•
Components are positioned relative to the top left corner
(0,0) of its container and are located using the container’s
coordinate space.

•
Components are added to a container using the
add
()
method.

12

Containers/Components

Containers
:

JFrame, equivalent to windows.

JPanel, applet’s display area is JPanel. It is often used to organize
components together in display.


Atomic Components:


JButtons (JCheckBoxes, JRadioButtons)


JComboBox


JLabels


JTextField (JTextArea)


Popup menus


ScrollPane

13

A Simple Example of Buttons

public class dummyButton extends JApplet {

public void init() {



setLayout(new FlowLayout(FlowLayout.CENTER));



bold = new JButton("bold");


add(bold);





add(italic = new JButton("italic"));


}



public void paint(Graphics g) {


super.paint(g);


g.setFont(new Font(MyFont, MyStyle, MySize));


g.drawString("Hello Java!", 50, 100);

}

………..

14

Java Event Handling Model

•
When user clicks a button or presses a key, an
event is generated. The button clicked is called
“event source”.

•
For each event source, you need to delegate a
Listener object to listen to it.

•
When the listener object hears an event, it
invokes a method to process the event.





button

Listener

Object

Event

bold = new JButton(“bold”);

bold.addActionListener(new BtnHandler());


public class BtnHandler implements ActionListener


{

…….


public void actionPerformed (ActionEvent e)


{


// code to handler start button

}

}

15


Make Buttons Responsive

The steps making buttons responsive:

1.
write an event handler which
implements interface
ActionListener
. Interface
ActionListener

has only one
method
actionPerformed
.

2.
implement method
actionPerformed

in the interface.

3.
add action listener (your event
handler) to each event source.

public class dummyButton extends JApplet {

public void init() {


add(bold = new Button("bold"));


bold.addActionListener (new butnhandler());


…. }


class butnhandler implements ActionListener {


public void actionPerformed (ActionEvent e) {


if (e.getSource() == bold)


style = Font.BOLD;


if (e.getSource() == italic)


style = Font.ITALIC;


repaint();


}


}

}

16

TextField

Textfiled is used for getting Text Input from users.

•
TextField

is constructed as:


JTextField tf = new
JTextField
(“input here”, 20);


add(tf);

•
an “action” event is generated when the user types “return” in a
TextField. You need to add an event
-
handler to the JTextField object by:


tf.
addActionListener

(new actionAdapter());

•
get the input text by:



String InputStr = tf.getText();

•
set the text field (for display) by:


tf.
setText
(“wrong input”);

17

TextArea

TextArea

is used to display or edit a number of rows and cols.

There is no event handler associated with it. It can be used

with ScrollPane to make it scrollable.

•
TextArea

can be constructed by:


JTextArea ta = new
JTextArea
(row, col);

•
display text in a TextArea by:


ta.
setText
(String str); OR


ta.
append
(String str);

•
get the whole text in
TextArea

by:


ta.
getText
();

18

Event handling for TextField

JTextField event is “action” type:

// jia/www/java/gui/textField.java

public class textFieldTest extends JApplet {

JTextField textin, textout;

JTextArea textArea;

public void init() {


setLayout(new FlowLayout());


add(textin = new JTextField("",20));


textin.
addActionListener
(



new TextHandler

());


textArea = new JTextArea(10,20);


add(textArea);


add(textout = new JTextField("initial",20));

}


class
TextHandler implements ActionListener

{


public void
actionPerformed
(ActionEvent e){


if (e.getSource() == textin) {


textArea.
setText
(textin.
getText
());


textout.setText(textin.getText());


}


}}}


19

GUI Event Handlings

•
Action Events
. Event sources:

–
JButton, List,

–
JTextField,

–
JMenuItem, etc.

•
Item Events
. Event sources:

–
JCheckbox,

–
JButtonGroup (RadioButton),

–
JComboBox

–
JChoice (Pop
-
up menu)

•
Mouse Events
.

–
Mouse press / release

–
Mouse drag / move

•
Key Events

–
Key press / release

–
Key typed

…………

20

CheckBox, ButtonGroup (RadioButton)

Checkbox

allows users to tick (true / false). A checkbox can be used independently.

•
a Checkbox is created by:

new
JCheckbox
();


ButtonGroup

is a group of checkboxes, only one of them can be selected.

•
a ButtonGroup is created by:

g

= new
ButtonGroup
();

•
a RadioButton is created & added to Group by:

btn

= new
JRadioButton
();

g
.add(
btn
);

// add
btn

to group
g


•
When a checkbox is clicked, an event of type
ItemEvent

is generated. A handler for
ItemEvent

type of events implements the interface
ItemListener
.

•
Interface
ItemListener

has only one method
itemStateChanged
(). You need to
implement it.

•
For
each checkbox
, an event handler (an object of type
addItemListener
) should
be added to it.

21

Choice

and Event Handler

•
Choice

is a drop
-
down menu of choices, created by


c =

Choice
().

•
A choice item is added in by:

c.
addItem
(“item”);

// c is a choice

•
An “item” event is generated when an item of the Choice is selected
and you can get the item by:

String e.
getItem
();

// e is the event

•
Choice event handler is the same as CheckBox, which implements
interface
ItemListener
.
ItemListener

has a method
itemStateChanged

(ItemEvent e).

•
An event handler object must be added to a Choice (the entire drop
-
down menu) by:


addItemListener
()

22

Mouse Event and Handler

•
Two interfaces,
MouseListener

and
MouseMotionListener
,
are for mouse events. Two adapters
MouseAdapter

and
MouseMotionAdapter
are provided for the interfaces. You
may either implement the interfaces or extend the adapters.

•
Mouse events include click, press, release, move, drag
(press
-
move), etc.

•
You need to implement all the methods in interfaces or
redefine (override) the
necessary

methods in adapters.

•
Add an Adapter object to the
window

in which you want to
catch mouse events.


23

Event Listener Interface and Adapter

•
For easy programming, some listener interfaces have
Adapter classes, which have default implementations of
all methods in the respective interfaces.

•
If the Adapter of a listener interface is provided, you don’t
have to implement the whole interface. You only need to
override some methods necessary to perform your work.



24

Example of Mouse Event Handler

//file:mouseDrawAline.java

import java.awt.event.*;

public class mouseDrawAline extends JApplet {


Point start = new Point(0,0), end = new Point(10,10);


public void init() {


addMouseListener
(new

myMouseAdapter());


addMouseMotionListener
(



new myMouseMotionAdapter());


}




public void paint(Graphics g) {



super.paint(g); // clear existing paintings



g.drawLine(start.x, start.y, end.x, end.y);


}







class myMouseAdapter
extends
MouseAdapter {


public void
mousePressed
(MouseEvent e) {




start.x = e.getX(); start.y = e.getY();


}


public void
mouseReleased
(MouseEvent e) {


end.x = e.getX(); end.y = e.getY();


repaint(); }


}


class myMouseMotionAdapter
extends




MouseMotionAdapter {


public void
mouseDragged
(MouseEvent e) {



end.x = e.getX(); end.y = e.getY();


repaint();


}

}


25

Keyboard Event

•
The interface for keyboard event handler is KeyListener.

•
KeyListener has an adapter KeyAdapter. A keyevent handler class can
either implement KeyListener interface, or extend KeyAdapter class.

•
Three methods defined in KeyListener:

–
keyPressed

(keyEvent), called when any key is pressed

–
keyTyped

(keyEvent), called only when a non
-
function key is pressed

–
keyReleased

(keyEvent), called when a key is released after keyPressed
or keyTyped event.

•
Three methods on keyEvent allows you to get keycode:

–
event.
getKeyCode
()

–
event.
getKeyChar
(), get printable letter of the key

–
event.
getKeyText
(keyCode), get string name of the key

26

Keyboard Event

public class keyTest extends JApplet {


public void init() {



setLayout (new BorderLayout ());


JTextArea textArea = new JTextArea( 30, 30 );


add("Center", textArea);


textArea.
addKeyListener
(new

myKeyAdapter());


}


class
myKeyAdapter

extends
KeyAdapter

{


public void
keyPressed
( KeyEvent event ) {


int keycode = event.getKeyCode();


String line = "Key pressed code: "+keycode;


line = line+ " & Text: "+

event.getKeyText(keycode);


textArea.setText(line+"
\
n");


}


public void
keyTyped
( KeyEvent event ){


textArea.append("Key typed: "+

event.getKeyChar()+"
\
n");


}



public void
keyReleased
(KeyEvent event ) {



textArea.append("Key released: "+

event.getKeyChar());


}



27

Event Processing Model in Java:

Event Types and Adapters

Java.lang.object

Java.awt.event.ComponenetAdapter

Java
.
awt
.
event
.
KeyAdapter


Java
.
awt
.
event
.
MouseAdapter


Java
.
awt
.
event
.
MouseMotionAdapter


Java
.
awt
.
event
.
WindowAdapter


Java
.
awt
.
event
.
ContainerAdapter


……………
..

28

Event Dispatching

•
Each component in a container is an object that has two methods,
dispathEvent
() and
processEvent
().

•
When an event is generated by an external device (mouse, kbd, etc), the out
-
most container’s
dispatchEvent
() is invoked, which dispatches the event
recursively layer by layer to the right component (i.e., event source).

•
When a component finds itself is the source of the event, it calls
processEvent
(e) to process the event.


//
dispatch an event to the right component

void
dispatchEvent
(AWTEvent e) {



……


if (newEventsOnly) {



if (eventEnabled(e)) {




processEvent
(e);



}

…….

}

29

Process Event

Method
processEvent
() of the component is the

root of all of the event
-
type processing functions.


protected void
processEvent
(AWTEvent e) {


if (e instanceof FocusEvent) {



processFocusEvent
((FocusEvent) e);


} elseif (e instanceof MouseEvent) {



switch(e.getId()) {



case MouseEvent.MOUSE_PRESSED:



case MouseEvent.MOUSE_RELEASED:



case MouseEvent.MOUSE_CLICKED:



case MouseEvent.MOUSE_ENTERED:


case MouseEvent.MOUSE_EIXTED:



processMouseEvent
((MouseEvent)e);



break;


case MouseEvent.MOUSE_MOVED:


case MouseEvent.MOUSE_DRAGGED:



processMouseMotionEvent
((Mouse
Event)e);



break; }


} else if (e instanceof KeyEvent) {



processKeyEvent
((KeyEvent)e);


} elseif (e instanceof ComponentEvent) {



processComponentEvent
((CompEve
nt) e); }

}


30

MouseMotion Event Handler (example)

•
Method
addMouseMotionListener
(
mymouseMotionAdapter
) does:


mouseMotionListener



mymouseMotionAdapter


•
Methods .mouseMoved or .mouseDragged are implemented by you.


protected void processMouseMotionEvent(MouseEvent e) {


if (mouseMotionListener != null) {



int id = e.getId();



switch(id) {



case Mouseevent.MOUSE_MOVED:




mouseMotionListener.mouseMoved
(e);




break;



case Mouseevent.MOUSE_DRAGGED:




mouseMotionListener.mouseDragged
(e);




break;



}


}

}

processEvent

processMouseMotionEvent

userHandlerObj
.mouseMoved

31

action

Event Processing

•
Button’s event handler
overrides

processEvent(). For
action

event, the
code in the right
-
hand box is
executed.

•
addActionListener
(myactionAdapter
)
does:


actionListener



myactionAdapter


protected void
processEvent
(AWTEvent e) {


if (e instanceof ActionEvent) {



processActionEvent
((Actionevent) e);



return;


}


super.processEvent(e);


}



processActionEvent
(ActionEvent e) {


if (actionListener != null)





actionListener.
actionPerformed
(e);


}

32

Window Layout

•
The default layout manager of content
-
panes is
BorderLayout
(); the default layout for JPanel is
FlowLayout
();

•
The default layout manager can be disabled by
setLayout
().

•
The common layouts are FlowLayout, BorderLayout, and
GridLayout.

•
Use JPanel to effectively organize components in
different layouts and then arrange the JPanels in top
-
layer
Layouts.

33

Flow Layout

•
FlowLayout
(align, hgap, vgap);

•
setLayout(new FlowLayout(FlowLayout.CENTER));

34

Border Layout

•
BorderLayout
()

•
To add components into a
container with BorderLayout,
the add method needs to specify
“North”, “South”, “East”,
“West”, or “Center”.

•
The component at the Center
automatically fills the whole
unoccupied rectangle area from
the center.

35

Grid Layout (Cont.)

•
GridLayout
(rows, cols, hgap, vgap)

•
When adding components into a frame with GridLayout, it fills the
row from left to the right before moving to the next row.


•
Example of displaying a calculator pad:


public class calculator extends JApplet {


public void init() {



setLayout(new GridLayout(3,4,5,5));



add(new Button(“0”));





……



add(new Button(“9”));


}

36

Use Panel to Organize Components

~jia/www/java/gui/layoutTest.java